Ethylene-α-olefin rubbers such as an ethylene-propylene copolymer rubber (EPR), an ethylene-propylene-diene copolymer rubber (EPDM) and the like have no unsaturated bonds in the main chain of its molecular structure, so that they are superior to general purpose conjugated diene rubbers in terms of heat and aging resistance, weatherability, and ozone resistance and widely used in applications to automobile components, electric wire materials, electric and electronic components, building and construction materials, industrial component materials and the like.
However, recent demand for higher performance and higher functionality of components is directed to further improvement of properties and to improvement of the processability more than ever because of complexity of the components and in order to reduce process cost.
Reduction of polymer viscosity is sufficient to improve the kneading processability of EPDM. However, drastic reduction of the molecular weight of EPDM to reduce the polymer viscosity lowers sealability of vulcanized rubber products obtained, resulting in a problem of being impractical. Also known is a method to broaden a molecular weight distribution and a composition distribution to reduce the polymer viscosity, but causes problems such that vulcanized rubber products obtained tend to have a sticky surface and poor low temperature properties.
Viscosity reduction of a compound is also sufficient to improve extrusion processability of EPDM, particularly an extrusion throughput and surface appearance of extruded products. However, blending a large volume of oil or decreasing the molecular weight of EPDM drastically to reduce the viscosity of the compound lowers sealability and strength of vulcanized rubber products obtained, resulting in a problem of being impractical.
On the other hand, increase in the viscosity of the compound is sufficient to improve shape retention during extrusion of EPDM, but causes a problem such that an extrusion throughput is reduced and surface appearance of extruded products is deteriorated.
From the background mentioned above, Patent Document 1 discusses improvement of sealability and strength of vulcanized rubber products obtained while keeping kneading processability and extrusion processability of EPDM. However, the improvement is not satisfactory in terms of balancing their processability with sealability and strength of EPDM.
Patent Document 1: Pamphlet of WO 00/59962